1. Field of the Invention
This invention relates to a process for forming a deposited film, particularly a process for forming an Al deposited film which can be preferably applied to electrodes or wiring of a semiconductor integrated circuit device, etc.
2. Related Background Art
In the prior art, in electronic devices or integrated circuits by use of semiconductors, for electrodes and wiring, aluminum (Al) has been primarily used. Al has many advantages such that it is inexpensive and high in electroconductivity, that it can be also internally chemically protected because a dense oxidized film can be formed on the surface, and that it has good adhesion to Si, etc.
As the method for forming Al film for electrodes and wiring of Al or Al alloy as mentioned above, there has been used in the prior art the sputtering method such as magnetron sputtering, etc. However, since the sputtering is generally the physical deposition method based on flying of sputtered particles in vacuum, the film thickness at the stepped portion or the insulating film side wall becomes extremely thin, leading to wire breaking in an extreme case. Nonuniformity of film thickness or wire breaking has the drawback that reliability of LSI is markedly lowered.
On the other hand, since the integration degree of the integrated circuit such as LSI, etc. is increased, and fine formation of wiring or multi-layer wiring has been particularly required in recent years, there is an increasing severe demand not found up to date for Al wiring of the prior art. With finer dimensional formation by increased integration degree, the surface of LSI, etc. is subject to excessive unevenness due to oxidation, diffusion, thin film deposition, and etching, etc. For example, electrodes or wiring metal must be deposited on the surface with a stepped difference, or deposited in a via-hole which is fine in diameter and deep. In 4 Mbit or 16 Mbit DRAM (dynamic RAM), etc., the aspect ratio (via-hole depth/via-hole diameter) of via-hole in which a metal composed mainly of Al such as Al, Al-Si, etc. is to be deposited is 1.0 or more, and the via-hole diameter itself also becomes 1 .mu.m or less. Therefore, even for a via-hole with large aspect ratio, the technique which can deposit a metal is required.
Particularly, for performing sure electrical connection to the device under insulating film such as SiO.sub.2, etc., rather than film formation, Al is required to be deposited so as to embed only the via-hole of the device. In such case, a method of depositing Al only on Si or metal surface and not depositing it on an insulating film such as SiO.sub.2, etc. is required.
Such selective deposition or selective growth cannot be realized by the sputtering method which has been used in the prior art. Since the sputtering method is a physical deposition method based on flying of the particles sputtered from the target in vacuum, the film thickness at the stepped portion or the insulating film side wall becomes extremely thin, leading even to wire breaking in an extreme case. And, nonuniformity of the film thickness and wire breaking will markedly lower reliability of LSI.
As the improved sputtering method, there has been developed the bias sputtering method in which a bias is applied on a substrate and deposition is performed so as to embed Al or an Al alloy only in the via-hole by utilizing the sputter etching action and the deposition action on the substrate surface. However, since the bias voltage of some 100 V or higher is applied on the substrate, deleterious influence on the device occurs because of charged particle damaging such as change in threshold of MOS-FET, etc. Also, because of presence of both etching action and deposition action, there is the problem that the deposition speed cannot be essentially improved.
In order to solve the problems as described above, various types of CVD (Chemical Vapor Deposition) methods have been proposed. In these methods, chemical reaction of the starting gas in some form is utilized. In plasma CVD or optical CVD, decomposition of the starting gas occurs in gas phase, and the active species formed there further reacts on the substrate to give rise to film formation. In these CVD methods, due to the reaction in gas phase, surface coverage on unevenness on the substrate surface is good. However, carbon atoms contained in the starting gas molecule are incorporated into the film. Also, particularly in plasma CVD, the problem remained that there was damage by charged particles (so called plasma damage) as in the case of the sputtering method.
The thermal CVD method, in which the film grows through the surface reaction primarily on the substrate surface, is good in surface coverage on unevenness such as stepped portion of the surface, etc. Also, it can be expected that deposition within via-hole will readily occur. Further, wire breaking at the stepped portion can be avoided.
For such reasons, as the formation method of Al film, the thermal CVD method has been variously studied. As the formation method of Al film according to general thermal CVD, there is used a method of transporting an organic aluminum dispersed in carrier gas to a heated substrate and pyrolyzing the gas molecules on the substrate to form a film. For example, in an example seen in Journal of Electrochemical Society, Vol. 131, p. 2175 (1984), by use of triisobutyl aluminum (i-C.sub.4 H.sub.9).sub.3 Al (TIBA) as organic aluminum gas, film formation is effected at a film formation temperature of 260.degree. C. and a reaction tube pressure of 0.5 torr to form a film of 3.4 .mu.ohm.cm.
Japanese Laid-open Patent Application No. 63-33569 describes a method of forming a film by using no TiCl.sub.4, but using in place thereof organic aluminum such as TIBA and heating it in the vicinity of the substrate. According to this method, Al can be deposited selectively only on the metal or semiconductor surface from which the naturally oxidized film has been removed.
In this case, it is clearly stated that the step of removing the naturally oxidized film on the substrate surface is necessary before introduction of TIBA. Also, it is described that, since TIBA can be used alone, no carrier gas is required to be used, but Ar gas may be also used as the carrier gas. However, the reaction of TIBA with another gas (e.g. H.sub.2) is not contemplated at all, and there is no description of use of H.sub.2 as the carrier gas. Also, in addition to TIBA, trimethyl aluminum (TMA) and triethyl aluminum (TEA) are mentioned, but there is no specific description of other organic metals. This is because, since the chemical properties of organic metals generally vary greatly if the organic substituent attached to the metal element varies little, it is necessary to investigate individually by detailed experimentation to determine what organic metal should be used.
In the CVD method as described above, not only there is an inconvenience that the naturally oxidized film must be removed, but also there is the drawback that no surface smoothness can be obtained. Also, there is the restriction that heating of the gas is necessary, and yet heating must be done in the vicinity of the substrate. Besides, it must also be experimentally determined at what proximity to the substrate heating must be done, whereby there is also the problem that the place for setting the heater cannot be necessarily freely chosen.
In the pre-text of the 2nd Symposium of Electrochemical Society, Branch of Japan (Jul. 7, 1989), on page 75, there is a description of film formation of Al according to the double wall CVD method. In this method, TIBA is used and the device is designed so that the gas temperature of TBA can be made higher than the substrate temperature. This method may be also regarded as a modification of the above-mentioned Japanese Laid-open Patent Application No. 63-33569. Also in this method, Al can be selectively grown only on a metal or semiconductor, but not only the difference between the gas temperature and the substrate surface temperature can be controlled with difficulty, but also there is the drawback that the bomb and the pipeline must be heated. Moreover, according to this method, there are involved such problems that no uniform continuous film can be formed, that flatness of the film is poor, that selectivity of Al selective growth cannot be maintained for so long time, etc., unless the film is made thick to some extent.
As described above, prior art methods cannot necessarily effect well selective growth of Al, and even if possible, there is a problem with respect to flatness, resistance, purity, etc. of the Al film formed. Also, there has been involved the problem that the film formation method is complicated and can be controlled with difficulty.